correctness batch: atomic writes, task refs, hmac, import-star, pickle

Per review §2:

- web/db.py: new _tx() context manager wraps multi-statement writers in
  BEGIN IMMEDIATE … COMMIT/ROLLBACK (our connections run in autocommit
  mode, so plain `with _lock:` doesn't give atomicity). partnership_accept
  (UPDATE + DELETE) and cleanup_retention (3 deletes/updates) now use it.
- Fire-and-forget tasks: add module-level _bg_tasks sets in web/app.py and
  web/enrichment.py. A _spawn() helper holds a strong ref until the task
  finishes so the GC can't drop it mid-flight (CPython's event loop only
  weakly references pending tasks).
- apply/main.py: require_api_key uses hmac.compare_digest, matching web's
  check. Also imports now use explicit names instead of `from settings *`.
- apply/language.py: replace `from settings import *` + `from paths import *`
  with explicit imports — this is the pattern that caused the LANGUAGE
  NameError earlier.
- alert/utils.py: pickle-based hash_any_object → deterministic JSON+sha256.
  Cheaper, portable across Python versions, no pickle attack surface.
- web/notifications.py: /fehler links repointed to /bewerbungen (the
  former page doesn't exist).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

web/enrichment.py

@@ -218,8 +218,20 @@ def _record_failure(flat_id: str, step: str, reason: str) -> None:
         pass
 
 
+# Holding strong references to every spawned task so asyncio doesn't GC them
+# mid-flight. create_task only weakly references tasks from the event loop.
+_bg_tasks: set[asyncio.Task] = set()
+
+
+def _spawn(coro) -> asyncio.Task:
+    t = asyncio.create_task(coro)
+    _bg_tasks.add(t)
+    t.add_done_callback(_bg_tasks.discard)
+    return t
+
+
 def kick(flat_id: str) -> None:
-    asyncio.create_task(asyncio.to_thread(enrich_flat_sync, flat_id))
+    _spawn(asyncio.to_thread(enrich_flat_sync, flat_id))
 
 
 async def _backfill_runner() -> None:
@@ -234,5 +246,5 @@ async def _backfill_runner() -> None:
 
 def kick_backfill() -> int:
     pending = db.flats_needing_enrichment(limit=200)
-    asyncio.create_task(_backfill_runner())
+    _spawn(_backfill_runner())
     return len(pending)